C Variable has incomplete initializer

Question

I am trying to make a struct with a default value, as described here: Default values in a C Struct. However, I have this C code, inside a header file:

/* tokens.h */
typedef struct {
    char *ID;
    char *KEY;
    char *TYPE;
} tokens;

const struct tokens TOKENS_DFLT = {
    "id",
    "key",
    "type"
};

And I am getting an error on line 7 saying:

error: variable 'TOKENS_DFLT' has initializer but incomplete type

Can anyone please explain to me what this problem is and how I can fix it and prevent it in the future?

Answer 1

You haven't defined struct tokens. You've defined an unnamed struct and simultaneously typedef-ed it to the type name tokens.

If you had instead defined

typedef struct tokens_ {
    char *ID;
    char *KEY;
    char *TYPE;
} tokens;

Then you could declare your constant as either:

const struct tokens_ TOKENS_DFLT = { ... };

Or

const tokens TOKENS_DFLT = { ... };

As it is, you're between two stools.

Answer 2

This:

const struct tokens TOKENS_DFLT = {
    "id",
    "key",
    "type"
};

should be:

const tokens TOKENS_DFLT = {
    "id",
    "key",
    "type"
};

Since you've defined the name tokens to mean struct tokens.